A Programmable Logic Controller (“PLC”) is a digitally operated electronic system, designed for use in an industrial environment, which uses a programmable memory for the integral storage of user-oriented instructions for implementing specific functions such as logic, sequencing, timing, counting, and arithmetic to control, through digital or analog inputs and outputs, various types of machines or processes. PLCs have long been used in heating, ventilation and air conditioning (“HVAC”) systems with actuators, dampers, valves, sensors, switches and other devices to control the movement and temperature of air, as well as to control other building functions.
In some cases, particularly when working on or troubleshooting the HVAC system, it is desired to allow manual control of one or more of the PLC outputs. U.S. Pat. No. 6,392,557, incorporated by reference, is directed to an override board having a manual auto/on/off switch on an insertable printed circuit board assembly for controlling an individual output of the programmable logic controller.